I wonder if crawford mama is right?--could it be his crop has a fistula?--or developed another pocket? Strange! --But thanks for posting the pictures! It always helps us all to learn the more we can experience.
Sounds like it is part of the crop. Don;t be messing with it as you don;t want to damage him. It does sound like a fistula. It would be something only a good avian vet could sort when he is bigger. So if he is healthy and its not bothering him in anyway then I wouldn;t worry about it too much. As long as he is eating ok and processing the food. I guess its just like us having an extra toe! He has an extra sack for his food. Just monitor it as he grows.
If its going up and down when he eats thats fine if it stays up and swells then you will need to get him looked at. As its not causeing pain and is not hard chances are its not a tumour more like a hurnia. Just don;t be proding it too much or you will make it sore! It may right itself as he grows or as I said a good Vet may need to put a few stitches in it when he is bigger crops are delicate so don;t mess with it and take him to see a vet who does deal with chickens if it gives him issues later.